Insulators are used to prevent the loss of electric charge or current from conductors in electric power transmission lines. A typical insulator is made from a material which has a very high resistance to electric current so that current flow through it is negligible. One type of insulator is referred to as a suspension insulator. It suspends a transmission line from an overhead support. A particular type of suspension insulator is referred to as a composite insulator.
A composite insulator may include a coated fiberglass rod surrounded by weathersheds made from a highly insulating material (see, for example, Kuhl U.S. Pat. No. 4,217,466, or, Bauer U.S. Pat. No. 4,246,696). A common approach has been to cover the fiberglass rod with an insulating coating. The weathersheds are molded from rubber or other polymeric material and then bonded to the coated rod using a polymer based adhesive. Metal fittings are provided at each end of the rod for connecting one end to a support pylon and the other end to a power line.
The weathersheds and the fiberglass rod are made from different material in order to enhance the performance of the two components in their distinct functions. In general, the rod takes the mechanical stress and contributes to the insulation function, while the weathersheds provide the majority of the insulation. The weathersheds also increase the distance which leakage currents must travel from one metal fitting to the other. The weathersheds must be fitted in such a way that no electric conduction can occur through the seam between a weathershed and the coated rod. Each weathershed must have good stability in all weather conditions. Furthermore, it must be of sufficient thickness to prevent its electrical resistance from being overcome. Additional information regarding the operation and construction of composite insulators may be found in U.S. Pat. Nos. 3,898,372; 4,267,403; 4,331,833; and 4,355,200.